Iraqi security forces on Monday launched a major offensive aimed at liberating the city of Mosul, the last major stronghold of Islamic State (IS) militants in Iraq.

Mosul, which is located some 400 km north of the Iraqi capital of Baghdad, has been under the group's control since June 2014, when Iraqi government forces abandoned their weapons and fled, enabling IS militants to take control of parts of Iraq's northern and western regions.

The joint forces of the Iraqi army, Peshmerga forces and allied paramilitary Sunni and Shiite Hashd Shaabi units, backed by the international and Iraqi air cover, have successfully recaptured nine villages from IS militants.

Iraqi Prime Minister Haider al-Abadi announced the offensive early Monday, saying that "the time of victory has come, and the moment of the great victory is approaching."

The offensive was supported by the United States with more than 100 US troops embedded in Iraqi and Kurdish forces to provide artillery support, intelligence, logistics and other support, according to the US Defense Ministry.

"This will be a key milestone in what I committed to doing when IS first emerged, which is we were going to roll them back and we are going to ultimately drive them out of population centers and we will destroy them and defeat them," said the US President Barack Obama.

Turkey, which has been locked in a row with Iraq about the presence of its troops at the Bashiqa camp in northern Iraq, joined airstrikes by the US-led coalition during the operation.

"Turkey has a 350-kilometer border with Iraq, and therefore cannot be expected to remain impartial to developments right across the border," Turkish President Recep Tayyip Erdogan said.

China and Iran also voiced their support for the Iraqi operation.

"China supports the Iraqi government's efforts to maintain national stability and fight terrorism. China hopes for national security and stability in Iraq at an early date," Chinese Foreign Ministry spokesperson Hua Chunying said at a regular news briefing.

"We will stand by the Iraqi government and nation until Iraq's full liberation (from terrorists) and the establishment of stability in the country," Iranian Foreign Ministry spokesman Bahram Qasemi said.

While Iraqi forces are moving gradually to encircle Mosul, humanitarian concerns arises as one million people may be forced to flee the terrorist group's last stronghold because of fighting.

"I am deeply concerned about the rapidly escalating humanitarian crisis in Mosul, Iraq, and the heightened threat to the health and lives of pregnant women who may be cut off from life-saving emergency obstetric care," Babatunde Osotimehin, the UN under-secretary-general and executive director of the UN Population Fund (UNFPA), said in a statement.

Among the 200,000 people likely to be displaced during the initial weeks of military operations in Mosul, an estimated 46,000 are women and girls of reproductive age, including about 8,000 who are pregnant or about to give birth, the statement said.

To cope with the possible humantarian crisis, Australia announced 7.5-million-US-dollars in humanitarian assistance to Iraq for the operation, Australian Foreign Minister Julie Bishop said on Tuesday.

The New Zealand government also said it would provide 1 million NZ dollars (718,600 US dollars) to help refuges from embattled Mosul.

The United Nations, for its part, said that the liberation of Mosul from IS terrorists must be followed by a dialogue conducive to peace and stability throughout the country.